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Secondary Electron D etector

Secondary Electron D etector. Ana Zehtabi Oskuie. SEM basics:. The electron beam interaction with near surface specimen atoms will make a signal which results in the SEM image . . Introduction Secondary electron secondary electron detector.

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Secondary Electron D etector

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  1. Secondary Electron Detector Ana ZehtabiOskuie

  2. SEM basics: The electron beam interaction with near surface specimen atoms will make a signal which results in the SEM image. IntroductionSecondary electron secondary electron detector

  3. IntroductionSecondary electron secondary electron detector

  4. Secondary electrons: Secondary electrons are generated from the collision between the incoming electrons and the loosely bonded outer electrons The scattered secondary electrons are categorized into three different types. IntroductionSecondaryelectronsecondary electron detector

  5. Electrons with energies E <50 e V: Secondary electrons (SE); (2) Electrons with energies 50 eV < E < EPE: Inelastically backscattered and elastically reflected electrons (RE). IntroductionSecondaryelectronsecondary electron detector

  6. SE1: First kind of secondary electrons are generated by the incoming electron beam as they enter the surface. The resulted image has high resolution which is only limited by the electron beam diameter. As SE1 is generated by near surface atoms so this kind of secondary electron is independent of V . acc IntroductionSecondaryelectronsecondary electron detector

  7. SE2: The secondary electrons that are generated by the backscattered electrons have returned to the surface after several inelastic scattering events SE2 come from a surface area which is bigger than the spot of the incoming electrons , also bigger than surface area of SE1. Therefore resolution is less than resolution of SE1 exclusively. IntroductionSecondaryelectronsecondary electron detector

  8. SE3: The generator of this kind is the SE2 that enters the lens surface and get scattered. Thus, it will result in a decrease of resolution. IntroductionSecondaryelectronsecondary electron detector

  9. Factors that affect Secondary Electron emission: 1- Atomic number (Z) • More SE2 are created with increasing Z IntroductionSecondaryelectron secondary electron detector

  10. 2- Beam energy and beam current • Electron yield goes through a maximum at low acc. voltage, then decreases with increasing acc. voltage IntroductionSecondaryelectronsecondary electron detector

  11. 3- The most important factor is the local curvature of the surface. If we have a edge more secondary electron can be emitted from specific spot. IntroductionSecondaryelectronsecondary electron detector

  12. How do we get an image?! 156 electrons! 288 electrons! Detector Image IntroductionSecondary electron secondaryelectrondetector

  13. Everhart-Thornley Detector • Secondary electrons (SE) are attracted to Faraday cage because of its positive charge. • Detector surface inside faraday cage (+12kV) accelerates electrons. • Scintillator layer gives off photons when struck by electrons. • Light travels down the light tube (LG) and hits photocathode and converted back to electrons. IntroductionSecondary electron secondaryelectrondetector

  14. Photomultiplier Tube: – It’s Entrance is coated with photocathode which has low work function, absorbs photons from scintillator and emits low-E photoelectrons. – SE repeatedly accelerated towards N successive dynodes biased at ~+100 V positive with respect to the last, producing more excess SE. (total increase can be as high as 106). IntroductionSecondary electron secondaryelectrondetector

  15. This amplification or gain, is controlled by the contrast control on the SEM. An increase in contrast is seen as a selective increase in the highlights of the image, rather then the shadows. The brightness of the image is controlled by the preamplifier or brightness control. In this case, both highlights and shadow areas of the image are amplified by the same amount. IntroductionSecondary electron secondaryelectrondetector

  16. Signal to noise ratio: Signal-to-noise ratio is defined as the power ratio between a signal and the background noise: Therefore, it would be a good way to measure image quality. IntroductionSecondary electron secondaryelectrondetector

  17. Factors that affect quality of image: backscattered electrons that have low energy cannot escape Faraday electric field and their absorption will generate a noise signal in our output. Electrical circuitries such as amplifier that are used in our detector produce noise that affects our output . In the diagram , as the beam scans from left to right, areas marked (B) will be bright because they are scanned by the beam and in the line of sight of the detector. Areas marked (I) are intermediate in brightness because they are out of the line of sight of the detector. IntroductionSecondary electron secondaryelectrondetector

  18. Regions D will be dark because they are not scanned with the beam at all. Tilting the specimen will alter the specimen topography relative to the beam and detector and may enhance or reduce image quality. IntroductionSecondary electron secondaryelectrondetector

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